1. Field of the Invention
The present invention relates to a communication system which employs a spread spectrum system, particularly, a direct sequence (DS) system.
2. Description of the Related Art
A spread spectrum communication system allows a plurality of users to share a single wide frequency band on the time and spatial basis upon executing communication. Attentions have been paid to such a spread spectrum communication system as a future communication system since it has a large system capacity per a bandwidth. However, when many terminals are installed in a communication network adopting such a spread spectrum communication system, a plurality of frequency channels (or time slots) are prepared and the number of communication signals to be retained in each frequency channel (or each time slot) is restricted, as in an ordinary communication system. Therefore, in order to start a new communication at each terminal, it is important to know in advance the number of spread spectrum signals that occupy the frequency channel (or the time slot) to be used.
In a common communication system, the CNR (Carrier to Noise Ratio) of a signal to be received is set equal to or greater than 10 dB. In such a common communication system, therefore, it is possible to determine whether or not a communication channel is occupied by detecting the level of the received carrier wave.
However, in the communication system employing the spread spectrum system, particularly, employing the DS system, the CNR of a received signal is used in the negative region. That is, in the communication system employing the DS system, a frequency bandwidth of the transmission signal is spread with a spread code over a frequency bandwidth that is sufficiently wider than a narrowest frequency bandwidth required for transmitting necessary information so that a power level of a transmission signal is set equal to or less than a noise signal level. Thus, the occupying state of a frequency channel (communication channel) cannot be detected by detecting the level of the received carrier wave (reception signal) in the spread spectrum communication system.
In order to detect the spread spectrum signal in the frequency channel at the receiver side in the spread spectrum communication system, it is necessary to conduct an inverse spread process with a spread code which is identical to the spread code used in the transmitter side. Since the received signal which is subjected to the spread spectrum process is processed with the inverse spread process to have a narrow bandwidth, the spread spectrum signal can be detected by detecting a signal level after the inverse spread process is conducted. Normally a spread code used in communication, i.e., a spread code used in the spread spectrum process of the transmission signal is associated with a specific number assigned to each terminal installed in the conventional spread spectrum communication system. Therefore, to detect the occupying state of the frequency channel at each terminal, it is necessary to allow each terminal to store the spread codes of all the terminals that utilize the same frequency channel. However, in the conventional spread spectrum communication system, it is to actually accomplish this detection. Further, the conventional spread spectrum communication system has a drawback that it is hard to keep a secret of communication content.
As described above, since it is difficult to detect the occupying state of a frequency channel in the conventional spread spectrum communication system, an interference signal is likely to be produced in communication between terminals in each system, thus disabling signal reception. This drawback is readily caused in an equal distributed system in which equal roles and functions are assigned to each terminal comprising the communication network.
FIG. 1 illustrates two spread spectrum communication systems of equal distributed type adjacent to each other. A communication system 1 comprises terminals 101, 102 and 103, and a communication system 2 comprises terminals 201 and 202. The terminals 101-103, 201 and 202 used in those systems 1 and 2 all have the same structure and functions, except for a system number specific to each system and a terminal number specific to each terminal. Those communication systems 1 and 2 may correspond to neighboring different users, for example, who purchased a plurality of terminals provided by the same manufacturer and may comprise independently their communication systems. In this case, it is assumed that the terminal 102 of the communication system 1 and the terminal 202 of the communication system 2 are used in close areas separated by a partition, a wall or the like. If the terminal 201 in the system 2 starts transferring information to the terminal 202 under the circumstance where the terminal 101 in the system 1 is transmitting information and the terminal 102 is receiving the information in FIG. 1, the transmission signal to the terminal 202 may also reach the nearby terminal 102 which is currently receiving the information and may be received as an interference signal. Under this situation, if the transmission distance between the terminals 101 and 102 is long and the received signal at the terminal 102 is weak, the communication at the terminal 102 may become disabled by the interference signal.
Since it is difficult to detect beforehand the occupying state of a frequency channel at each terminal in the conventional spread spectrum communication system, an interference signal is easily produced in communication between terminals of the system. If the transmission distance is large and the received signal is weak, in particular, the signal communication is disabled.